Process for adhering a surface layer to a polyolefin substrate and use of a tie layer for adhesion

Abstract: Improved adhesion to a polyolefin substrate, especially polypropylene, of surface layers of acrylic-based or other protective polymers in the form of a plastic film, sheet, or coating or a latex or solvent-based paint, ink, or coating is achieved by use of an intermediate layer, laminate, or veneer of a blend of segmented copolymer of a polyolefin and a poly(methacrylate) which is blended with a core/shell copolymer having a crosslinked rubber core.

Description: This invention is concerned with a process for adhering a surface layer to a polyolefin substrate and the use of a tie layer. The surface layer may be an acrylic-based or other protective polymer which is in the form of a plastic film, sheet, or coating or a latex or solvent-based paint, ink, colourant or coating. The invention may be further applied to improve adherence or adhesion of pressure-sensitive adhesives which may be used to bond cloth, carpet, non-woven fabrics, and plastics to a substrate. Matrix polymers, such as ABS, may be used as substrates for acrylic plastic sheet, film, coatings, inks, or latices, where the latter impart weatherability, decorative effects and printability. The matrix polymers are useful in the assemblage of automobiles and other vehicles, such as automotive interiors, trunks (boots), truck (lorry) cabs, railroad car interiors and van (caravan) trim, equipment housing and furniture, where they offer advantages in cost and formability over metal or wood. Unfortunately, when an inexpensive matrix polymer, for example, a polyolefin such as polypropylene or polyethylene, is used, the acrylic polymers do not adhere well without special treatment, such as corona discharge or other oxidative treatments of the substrate. Acceptable adhesives or pre-coatings for the polyolefin prior to application of the acrylic polymer are also not readily available. Acrylic polymers, more preferably polymers of methyl methacrylate, are useful for this purpose but are expensive and not as tough as certain plastic substrates used in the automotive and other industries. Such acrylic polymers are useful to achieve a combination of adhesion, toughness, and lower cost when combined with a cheaper, tougher polyolefin in a laminar structure (which structure may require a laminating adhesive in the case of polyolefins to prevent delamination), or when the surface of the polyolefin is altered to promote adhesion. Thus the art has sought means for adherence of various forms of (meth)acrylic polymers to inexpensive polyolefin substrates as a means for protecting, coloring, or adhering to such polyolefins, without the use of expensive extemal physical treatment such as corona discharge. There are several ways of causing adhesion between a non-polar substrate, such as a polyolefin, and a paint, coating, or ink. One is a tie layer between the polyolefin and the surface layer which must develop adequate adhesion. US-A- 4,957,974 EP 0 335 649 discloses the use of specific graft copolymers of polyolefins and polymethacrylates as tie layers between otherwise incompatible polymers. US-A-5,035,933 discloses the use of such graft copolymers as tie layers between polyolefins and certain barrier polymers. Unfortunately, these graft or segmented copolymers, either by themselves or when blended with polyolefins, when based on a methyl methacrylate graft, have not been satisfactory in their ability to accept a paint with good adhesion against standard tests. EP 0431733 discloses improvement in the compatibility of blends of polyolefin and core-shell polymers by the addition of polyolefin-acrylic graft copolymers. There is no teaching that the resultant mixture is a useful tielayer between otherwise imcompatible polymers. The aim of the present invention is to provide a process for adhering a surface layer to a polyolefin substrate comprising: a) forming a tie layer comprising a blend of: (i) from 5 to 95 parts per 100 parts of tie layer of a multi-stage polymer containing a crosslinked core, wherein the core is formed from at least 50% of at least one of butadiene or butyl acrylate, and wherein the second-stage is formed from at least 50 % methyl methacrylate, and wherein the ratio of first-stage to second-stage polymer is at least 3/2; and (ii) from 95 to 5 parts per 100 parts of tie layer of a segmented copolymer having at least one segment of a non-polar polyolefin formed from at least 50% of ethylene or propylene and having at least one polar segment of a polymer formed from at least 80 weight percent of an alkyl methacrylate, the weight-average molecular weight of all segments being above 20,000, the ratio of polar segments to non-polar segments being from 9:1 to 1:4. b) applying the tie layer under heat and pressure to the polyolefin substrate; c) separately and concurrently applying the surface layer to the tie layer; and d) where necessary, drying the surface layer to produce an adherent film. The polyolefin substrate is preferably polypropylene, and is more preferably glass-filled polypropylene sheet. The tie-layer is preferably in the form of a film or sheet and is either co-injection molded or co-extruded with the polyolefin. The surface layer may be a (meth)acrylic film or laminate, a paint, an ink or a coating. The invention also provides use of a tie layer for adhering a (meth)acrylic film, a paint, an ink or a coating to a polyolefin substrate. The tie layer comprises a blend of : (i) from 5 to 95 parts per 100 parts of tie layer of a multi-stage polymer containing a crosslinked core, wherein the core is formed from at least 50% of at least one of butadiene or butyl acrylate, and wherein the second-stage is formed from at least 50% methyl methacrylate, and wherein the ratio of first-stage to second-stage polymer is at least 3/2; and (ii) from 95 to 5 parts per 100 parts of tie layer of a segmented copolymer having at least one segment of a non-polar polyolefin formed from at least 50% of ethylene or propylene and having at least one polar segment of a polymer formed from at least 80 weight percent of an alkyl methacrylate, the weight-average molecular weight of all segments being above 20,000, the ratio of polar segments to non-polar segments being from 9:1 to 1:4. The present invention allows combinations of inexpensive polyolefins to be protected, painted, printed, or adhered to in a manner to replace more expensive matrix polymers. Further, in many cases, no additional adhesive need be employed. In the following text, "film" refers to a continuous plastic matrix less than about 0.25mm (10 mils units) thick, whilst "sheet" refers to a continuous plastic matrix more than 0.25mm (10 mils units) thick. "Predominantly" has the meaning of at least 50%. A preferred first multi-stage polymer is that described in US-A- 5,237,004. The polypropylene may contain the additive described in US-A- 4,957,974 for the purposes of improving its melt processing and sag resistance during thermoforming. The tie coat may be applied by methods known to the art. Such methods include extrusion of the tie-coat on to either the acrylic polymer or the polypropylene, either as a co-extrudate or as an already formed film, followed by application of the remaining polymer with appropriate temperature and pressure conditions to optimize lamination. Lamination of all three components simultaneously, such as by co- extrusion or co-injection molding of all components, is also possible. The first film or sheet may be made by conventional means, such as extrusion, calendering and compressing molding. It may be applied to the polyolefin film or sheet by compression in a press with heat, for example, by application through heated rolls. Continuous processes for extruding one or both films or sheets, contacting, separating, and re-using the first film or sheet in contact with additional polypropylene may be envisaged. It should be noted that component (i) of the blend may form a film or sheet under careful processing conditions, but it is very difficult to form such a film or sheet by the variety of molding conditions which may be utilized for forming the blend of components (i) and (ii). Further, such a film or sheet solely from component (i) will not laminate adequately to the polyolefin sheet or film. The addition of component (ii) aids in lamination to the polyolefin but it also serves as a carrier or flow aid for component (i) so the blend can be injection molded or extruded under a variety of conditions. A film solely from component (ii) can be formed fairly readily and will exhibit adequate adhesion to the polyolefin; it is deficient, however, in exhibiting acceptable adhesion to the (meth)acrylic film or laminate or to a latex, paint, or ink. Component (i) is required to promote that required adhesion. The resulting laminates may be painted, adhered to carpet or printed. They may be thermoformed into useful objects of commerce and transportation, such as machine housings, containers and automotive parts. Printability to polyolefin injection molded parts or extruded or blown polyolefin films may be improved for use in containers and bags. Paintability of exterior automobile parts, such as fenders, spoilers, rocker panels and blow-molded bumpers, may be improved. Injection molded substrates of polypropylene will exhibit better adhesion to foam-in-place polyurethane foam. Polyolefin pipe, when so modified, may be a more adherent surface for printing, labelling and application of coatings, such as insulation. Printability of synthetic paper based on polyolefins may be enhanced, as may be the adhesion of printable polyolefin films to plastic containers, where the labels are added after molding. The concept will be useful in packaging with bi- and multilayer composite films of polyolefins. Construction of circuit boards on to a polypropylene substrate may be enhanced when the substrate is modified with an acrylic surface. COMPARATIVE EXAMPLE 1-- Process for Tie-Layer Film or Sheet This example describes one way in which a tie-layer may be utilized, although it is not the tie-layer of the present invention. A polymer, preferably made by the process of US-A- 5,247,024, of the composition of Example 69 of US-A-5,237004 is processed into a 0.254mm (10 mil). film by compression molding the polymer (240 degrees C., ca.20,000kg (20 metric tons)). The film is then used as a tie-layer in a laminate between films prepared in a similar manner of (a) a polypropylene homopolymer of MFR 0.8, and (b) a poly(methyl methacrylate) of MW ca. 125,000. Here testing for adhesion is conducted as follows: a polypropylene floc carpet backed with a pressure-sensitive adhesive believed to be a butadienestyrene polymer combined with a separate layer of the glycerol ester of colophony rosin is adhered to various plastics substrates under a pressure of 2000 psi ( 14000 kPa). Samples are oven-aged at 39 degrees C. for 12 hours, and then tested within 5 minutes of removal from the oven at 50% relative humidity on a Zwick tensile tester at 300 mm. /min. peel rate. The adhesion to the composite is equivalent to that for an acrylic monolithic substrate, surpassing the performance of either polypropylene or ABS. No delamination at the surface or between acrylic and polypropylene is noted. EXAMPLES 2 - 5 Preparation of a Blended Film with Enhanced Adhesion to Polypropylene Films are prepared by dry-blending powders in ratios of 9:1 and of 1:9 of a butyl acrylate/methyl methacrylate core-shell polymer of particle size 8 microns of the composition of Example 69 of US-A-5,237004 (Modifier A) and a segmented polypropylene/poly(methyl methacrylate/butyl acrylate copolymer (95/5) polymer containing 56 weight % polypropylene, prepared by the process of Example 7 of US-A- 4 957974 (Carrier A). The dry-blended powders are melt-blended and extruded into pellets at 204 - 218 °C. Films are compression-molded from pellets to a thickness of 5 - 20 mils (0.12 - 0.50 mm.) at 204 - 210 °C for one minute at 14 - 140 mPa, and then laminated as in Example 3 to a polypropylene surface. Excellent retention of a solvent-based lacquer (100%) is observed, using the ASTM D 3359-78 cross-hatch test. Testing results on the blend film without lamination to a polypropylene substrate are mixed; excellent retention of a solvent-based lacquer (100%) is observed, using the ASTM D 3359-78 cross-hatch test. With an automotive polyurethane paint, in one test facility poor adhesion is observed, but in another excellent retention of adhesion is seen for both the automotive polyurethane and a melamine polyester. It is noted that these blends have rheological properties acceptable for co-injection molding with polyolefins, that is, the melt properties of the blend at temperatures close to the temperature at which the polyolefin base is injection-molded show a higher melt viscosity than the polyolefin base, but not so high that co-processing cannot occur. In a similar manner, the blended films are laminated to a thermoplastic polyolefin (TPO), and excellent adhesion of a solvent-based lacquer paint is observed by the same ASTM test. When a film formed solely from the composition of Example 69 is tested as a control, no adhesion of the paint is observed in any of the tests. EXAMPLE 6-- Preparation of a Blended Film for Adherence to a Thermoplastic Polyolefin A film is prepared from a polypropylene modified with an ethylenepropylene-diene elastomer. The films taught in Example 2 are laminated to the impact-modified polypropylene. Good adherence of a solvent-based lacquer paint (95% and 98% retention) is observed to the laminated veneer which adheres well to the impact-modified polypropylene substrate. However, when the ratios of carrier/additive are extended, it is seen that outstanding adhesion of the solvent-based acrylic paint is found only at the ends of the compositional range. When a film formed solely from the composition of Example 69 is tested as a control, no adhesion is observed in the tests. Additive Ratio, Carrier/Additive Cross-Hatch, % Retention (solvent-based acrylic) A 9/1 95 A 8/2 20 A 7/3 20 A 6/4 20 A 5/5 0 A 4/6 15 A 3/7 40 A 2/8 55 A 1/9 98 EXAMPLE 7--Further Examples of Improved Paintability of Thermoplastic Polyolefin In these Examples, other multistage core/shell polymers were used to achieve a similar effect. Modifier B is a core/shell polymer based on a cross-linked butyl acrylate core (75 parts) with 25 parts of a methyl methacrylate/acryloxypropionic (95/5) copolymeric shell . Modifier C is a core/shell polymer based on a crosslinked butadiene/styrene core staged first with styrene and then with methyl methacrylate, stabilized with antioxidants. It is noted that these blends have rheological properties acceptable for co-injection molding with polyolefins, that is, the melt properties of the blend at temperatures close to the temperature at which the polyolefin base is injection-molded show a higher melt viscosity than the polyolefin base, but not so high that co-processing cannot occur. They were blended with Carrier A at various ratios, laminated to a thermoplastic polyolefin based on polypropylene and ethylene/propylene rubber, treated with a urethane coating, and then tested for adhesion of the coating to the veneer or laminate formed from the blend. Results are shown below. When a film formed solely from the composition of Example 69 is tested as a control, no adhesion is observed in any of the tests. Additive Ratio, Carrier/Additive Cross-Hatch, % Retention B 9/1 6 B 8/2 0 B 7/3 0 B 6/4 4 B 5/5 17 B 4/6 40 B 3/7 93 B 2/8 100 B 1/9 100 C 9/1 91 C 8/2 96 C 7/3 25 C 6/4 25 C 5/5 15 C 4/6 20 C 3/7 2 C 2/8 30 C 1/9 96 EXAMPLE 8--Treatment of Various Blends of Core/shell Polymers/Segmented Copolymers directly with Coatings In this example, direct painting by an automotive polyurethane is done on the blends without lamination to the TPO substrate. Additive Ratio, Carrier/Additive Cross-Hatch, % Retention B 2/8 100 C 1/9 100 --- 100/0 0

Claims: A process for adhering a surface layer to a polyolefin substrate comprising the steps of a) forming a tie layer comprising a blend of: (i) from 5 to 95 parts per 100 parts of tie layer of a multi-stage polymer containing a crosslinked core, wherein the core is formed from at least 50% of at least one of butadiene or butyl acrylate, and wherein the second-stage is formed from at least 50% methyl methacrylate, and wherein the ratio of first-stage to second-stage polymer is at least 3/2; and (ii) from 95 to 5 parts per 100 parts of tie layer of a segmented copolymer having at least one segment of a non-polar polyolefin formed from at least 50% of ethylene or propylene and having at least one polar segment of a polymer formed from at least 80 weight percent of an alkyl methacrylate, the weight-average molecular weight of all segments being above 20,000, the ratio of polar segments to non-polar segments being from 9:1 to 1:4. b) applying the tie layer under heat and pressure to the polyolefin substrate; c) separately and concurrently applying the surface layer to the tie layer; and d) where necessary, drying the surface layer to produce an adherent film. Process of claim 1, wherein the segmented copolymer is a segmented copolymer of polypropylene and a polymer containing at least 80 weight percent of units of methyl methacrylate. Process of claim 1 or 2 wherein the tie layer is a film which is in the form of a continuous plastic matrix of thickness less than 0.25 mm (10 mils) or a sheet which is a continuous plastic matrix of thickness more than 0.25 mm (10 mils). Process as claimed in any of claims 1 to 3, wherein the polyolefin substrate is polypropylene. Process as claimed in claim 4, wherein the polypropylene is a glass-filled polypropylene sheet. Process of any of claims 1 to 5 wherein the tie layer is either co-injection molded or co-extruded with the polyolefin substrate Process of any one of claims 1 to 6 wherein the surface layer is a (meth)acrylic film or laminate, a paint, an ink or a coating. Use of a tie layer comprising a blend of: (i) from 5 to 95 parts per 100 parts of tie layer of a multi-stage polymer containing a crosslinked core, wherein the core is formed from at least 50% of at least one of butadiene or butyl acrylate, and wherein the second-stage is formed from at least 50% methyl methacrylate, and wherein the ratio of first-stage to second-stage polymer is at least 3/2; and (ii) from 95 to 5 parts per 100 parts of tie layer of a segmented copolymer having at least one segment of a non-polar polyolefin formed from at least 50% of ethylene or propylene and having at least one polar segment of a polymer formed from at least 80 weight percent of an alkyl methacrylate, the weight-average molecular weight of all segments being above 20,000, the ratio of polar segments to non-polar segments being from 9:1 to 1:4. for adhering a (meth)acrylic film, a paint, an ink or a coating to a polyolefin substrate.

Inventors: GOLDMAN THEODORE DANIEL, HARVEY NOEL GRAY, HESLER CARL MICHAEL, GOLDMAN, THEODORE DANIEL, HARVEY, NOEL GRAY, HESLER, CARL MICHAEL
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1994
Main classification: C08L51
IPC codes: H05K1, H05K3, C08L51, C08L9, C08L21, B32B27, C08L33, C08L53, C08J5